1. Field of the Invention
The invention relates to a method for producing a delivery module having an electric PTC heater, which delivery module can be installed into a tank. The delivery module is suitable, in particular, for tanks, in which a liquid additive (in particular, urea/water solution) is stored.
2. Related Art
Exhaust gas treatment apparatuses are known, into which a liquid additive is fed for exhaust gas purification. In exhaust gas treatment apparatuses of this type, for example, the method of selective catalytic reduction (SCR method) is carried out. In this method, nitrogen oxide compounds in the exhaust gas of an internal combustion engine are removed with the aid of a reducing agent. In particular, ammonia is used as reducing agent. Ammonia is normally not directly stored in motor vehicles, but rather in the form of a liquid additive which can be converted to form ammonia within the exhaust gas (in the exhaust gas treatment apparatus) and/or outside the exhaust gas (in a reactor which is provided specially for this purpose). A liquid additive which is used particularly frequently for the exhaust gas purification is urea/water solution. A urea/water solution having a urea content of 32.5% is available under the commercial name AdBlue®.
During the design of delivery modules and tanks for providing, in particular, aqueous additives, it is to be taken into consideration that they can freeze at low temperatures. For example, a urea/water solution freezes at approximately −11° C. In the automotive sector, low temperatures of this type can occur, in particular, during long standstill phases of the motor vehicle in winter. This is a problem, in particular, during restarting of the delivery module. There is regularly the requirement that liquid additive is to be available immediately during starting of the motor vehicle. For this reason, it is known to provide an (active) heating system on/in a tank for storing the liquid additive or on/in the delivery module. Electric heaters, heaters which are operated with heated cooling liquid of an internal combustion engine and/or heaters which utilize the heat of the exhaust gases of an internal combustion engine (exhaust gas heat) have been proposed for this purpose.
Electric heaters have the advantage that they can already provide a great quantity of heating energy even very briefly after the operating start of a motor vehicle. In contrast, heated cooling liquid and exhaust gas heat are available only after a relatively long operating phase of an internal combustion engine. Electrical energy has to be capable of being provided, however, in a sufficient quantity by an energy store (for example, a rechargeable battery or a capacitor). The possibility of providing electrical energy in a motor vehicle is limited firstly with regard to the overall available quantity of energy. For example, only a defined energy quantity (for example, 1 or 2 MJ) can be made available overall for heating. Moreover, a limit typically exists with regard to the electrical power which can be demanded. This limit results from the capabilities of the energy store and/or the electric connecting lines from the energy store to the heater.
During the design of heating systems of this type for tanks and/or delivery modules for providing liquid additives, it is to be taken into consideration, moreover, that the liquid additive can be influenced chemically by way of excessive heating. The urea/water solution is converted chemically, for example, to form ammonia or undesired intermediate products if a limit temperature is exceeded. This should not occur in the delivery module and tank because the ammonia might corrode and damage components of the delivery unit. Therefore, heaters which are automatically deactivated when a maximum temperature is exceeded are particularly advantageous. Electric PTC heating elements (PTC=positive temperature coefficient) are known, for example. They are electric heating elements which exhibit a particular dependence of the electrical resistance on the temperature. In various PTC heating elements, there is in each case a characteristic and/or material-specific switching temperature, at which the electrical resistance is increased suddenly. Therefore, the heating current through a PTC heating element is reduced when the switching temperature is reached, and it is prevented that the temperature rises significantly above the switching temperature. A material which has the described PTC properties and of which most PTC heating elements consist is, for example, barium titanate (BaTiO3). It is to be noted, however, that the available great number of different PTC materials which can be used here also have different prices.